1. Field of the Invention
The present invention relates to a plasma processing apparatus using an antenna for producing high frequency energy fields in a plasma chamber.
2. Description of the Related Art
The plasmas used in conventional plasma assisted etching are surface wave (Whistler wave) plasmas (SWP), electron cyclotron resonance plasmas (ECR) and induction coupled plasmas (ICP). As disclosed in Japanese Laid-Open Patent Specification Hei-3-68773, the plasma processing apparatus of the SWP method includes an antenna surrounding the upper part of a plasma chamber. The antenna is connected to a high frequency source (several tens of megahertz) to produce a Whistler wave in the plasma generation chamber. A solenoid is provided for producing a magnetic field in a direction parallel to the vertical axis of the upper part of the plasma chamber. The lower part of the chamber is a plasma dispersion area which is surrounded by a plurality of parallel permanent magnets co confine electrons. A wafer platen is supported in the lower part of the chamber and connected to an RF bias oscillator.
The ECR etching method is one in which microwave plasma etching occurs in a discharge chamber utilizing a resonance between the cyclotron motion of electrons in magnetic and microwave fields, as typically disclosed in Japanese Laid-Open Patent Specification Sho-60 -134423.
The ICP method, as disclosed in Japanese Laid-Open Patent Specification Hei-3-79025, uses a spiral coil mounted on a plasma chamber. High frequency energy (several tens of megahertz) is applied to the coil and a wafer platen placed in the chamber to produce high frequency electromagnetic fields.
Although high-density plasmas can be obtained by these prior art techniques, there is one disadvantage in that an excessively great number of active (high energy) electrons are produced in a plasma and the reactive gases are excessively dissociated by the active electrons, so that the plasma is maintained at high electron temperatures. Because of the excessive dissociation, the selectivity (defined as the ratio of etch rates between different materials) of plasma assisted etching on a silicon substrate through a silicon-dioxide mask is as high as 12:1. Due to the high electron temperatures, a large amount of electrons accumulate on the sidewalls of holes in the silicon-dioxide mask and electrons impinging through the holes tend to diverge as they move past the holes into deeper areas of etched silicon substrate, producing a difference in pattern width between the mask and the substrate (i.e., low selectivity). As a result, etch rate must be varied with pattern width in order to obtain a high selectivity.